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We examine fault-related rocks from the upper parts of the active San Andreas and ancient San Gabriel Faults, southern California, to determine the nature and origin of micro-scale composition and geochemistry of fault-related rocks. These data constrain the nature and extent of fluid-rock interactions and processes by which slip is accommodated on shallow portions of these faults. The steeply dipping San Gabriel Fault (SGF) was sampled in a steeply inclined borehole to a depth of 400 m, and the San Andreas Fault (SAF) was sampled by seven northeast-plunging boreholes to a depth of 250 m. Fault damage zones 100m+ wide exhibit narrow fault core zones within broad damage zones. Petrographic, mineralogic, whole-rock geochemical analyses and synchrotron-based X-ray fluorescence mapping of whole drill core and thin sections reveal evidence for repeated syntectonic hydrothermal alteration, Fe-Mn rich mineralization, shearing, and brecciation that resulted in the formation of foliated cataclasites and clay and chlorite-rich shear zones in fractured network fault zones. Mineralization and alteration include clay and chlorite development, carbonate and zeolite mineralization, and the mobility of trace and transition elements in the deformed rocks. Textural evidence for repeated shearing, alteration, vein formation, brittle deformation fracture, fault slip, pressure solution, and re-lithification of faulted rocks suggests that hydrothermal alteration occurred during deformation at shallow levels. The rock assemblages likely represent significantly weakened rocks that have the potential to slip at low shear stresses, experience creep, distribute seismic energy within and near the fault, and show that hydrothermal conditions in faults may exist at very shallow levels of active faults.

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